Pacemakers and other cardiac rhythm management devices deliver cardiac therapy to a patient's heart to assist in obtaining a rhythm of heart contractions that maintains sufficient blood flow through the patient's circulatory system under a variety of conditions. In particular, rate-adaptive pacemakers deliver electrical pacing pulses to stimulate contractions of the heart. The rate at which the pulses are delivered is adjusted to accommodate a metabolic need of the patient. During exercise, higher pacing rates are delivered, while lower pacing rates are delivered when the patient is at rest.
Different parameters are used as an indication of the patient's metabolic need for pacing therapy, including: blood pH, blood temperature, electrocardiogram (ECG) artifacts such as QT interval, blood oxygen saturation, breathing rate, minute ventilation, etc. Pacemakers include specific control algorithms for tracking the parameter indicating metabolic need, and providing a control signal for adjusting the pacing rate accordingly. A variety of difficulties exist that complicate sensing of the parameter indicating metabolic need and controlling the pacing rate.
For example, detecting blood pH encounters sensor stability problems. pH sensors may drift with age and time. Blood oxygenation saturation is measured using light emitters that complicate the lead system used to couple the pacemaker's pulse generator to the heart. Blood temperature is a poor indicator of metabolic need because of the long time lag between the onset of exercise and any detectable increase in blood temperature. ECG artifacts, such as QT interval, are difficult to detect in the presence of other myopotentials and motion artifacts. Breathing rate, also referred to as respiratory rate, is not particularly well correlated with the need for increased blood circulation. For example, it is possible for respiratory rate to increase while the patient is sleeping or talking.
Minute ventilation (also referred to as "minute volume" or "MV") is a respiratory-related parameter that is a measure of the volume of air inhaled and exhaled during a particular period of time. Minute ventilation correlates well with the patient's metabolic need for an increased heart rate over a range of heart rates. A minute ventilation signal can be obtained by measuring transthoracic (across the chest or thorax) impedance. Transthoracic impedance provides respiratory or ventilation information, including how fast and how deeply a patient is breathing. A component of transthoracic impedance varies as the patient inhales and exhales. Ventilation (e.g., breathing rate, which is also referred to as "ventilation rate" or "VR", and breathing volume, which is also referred to as "tidal volume" or "TV") information is included in the impedance signal. A minute ventilation signal (also referred to as "minute volume" or "MV") signal is derived from the impedance signal, as illustrated by Equation 1. MV measures air flow rate (e.g., liters per minute), TV measures volume per breath (e.g., liters per breath), and VR measures breathing rate (e.g., breaths per minute). EQU MV=TV.times.VR (1)
A larger MV signal indicates a metabolic need for an increased heart rate, and the pacing rate can be adjusted accordingly by a cardiac rhythm management device. For example, one approach for measuring transthoracic impedance is described in Hauck et al., U.S. Pat. No. 5,318,597 entitled "RATE ADAPTIVE CARDIAC RHYTHM MANAGEMENT DEVICE CONTROL ALGORITHM USING TRANS-THORACIC VENTILATION," assigned to the assignee of the present application, the disclosure of which is incorporated herein by reference. However, many problems must be overcome to provide the most effective cardiac rhythm management therapy to the patient in a device that can remain implanted in the patient for a long period of time before requiring a costly surgical explantation and replacement procedure.
First, ventilation information included in the transthoracic impedance signal is confounded with a variety of extraneous signals that makes the ventilation information difficult to detect. For example, as the heart contracts during each cardiac cycle, its blood volume changes, contributing to a significant change in the transthoracic impedance signal that is unrelated to the ventilation information. The change in the transthoracic impedance signal due to blood volume changes resulting from heart contractions is referred to as cardiac "stroke volume" or "stroke" signal. Moreover, the frequencies of the heart contractions (e.g., 1-3 Hz) are extremely close to the frequency of the patient's breathing (e.g., under 1 Hz). This complicates separation of the stroke signal and the ventilation signal.
Furthermore, the frequency of the stroke and ventilation signals changes according to the patient's activity. For example, a resting patient may have a heart rate of 60 beats per minute and a ventilation rate of 10 breaths per minute. When exercising, the same patient may have a heart rate of 120 beats per minute and a ventilation rate of 60 breaths per minute. The changing frequencies of the stroke and ventilation signals further complicates the separation of these signals.
Another aspect of heart contractions also masks the ventilation signal. Heart contractions are initiated by electrical depolarizations (e.g., a QRS complex) resulting from paced or intrinsic heart activity. Such electrical heart activity signals may be detected during the measurement of transthoracic impedance. This further diminishes the accuracy of the transthoracic impedance measurement, and increases the difficulty of obtaining accurate ventilation information.
A further problem with certain other minute ventilation based cardiac rhythm management devices results from the use of a relatively high amplitude current pulse (e.g., 1 milliampere) to detect transthoracic impedance. Using high amplitude stimuli wastes power, risks capturing the heart (i.e., evoking a contraction), may trigger false detection of intrinsic heart activity by the pacemaker's sense amplifiers, and may produce a confusing or annoying artifact on electrocardiogram (ECG) traces or other diagnostic equipment.
Thus, there is a need for a cardiac rhythm management device that effectively manages the patient's heart rate based on an accurate indication of metabolic need. Such a cardiac rhythm management device must be sufficiently robust to operate in the presence of extraneous noise signals that confound the indication of metabolic need. There is a further need for such a device to operate at low power consumption, in order to maximize the usable life of the battery-powered implantable device.